When a flexible metal-clad laminate (hereinafter referred to as "flexible metal-clad laminate") is etched to remove unnecessary portions of a conductive material from the board, a printed circuit board can be formed. On this printed circuit board, various electronic parts such as semiconductors are mounted, and the board is then incorporated into an electronic apparatus.
The mounting is to attach parts to the board by the use of soldering, screwing or the employment of wiring materials. Examples of the mounting means include parts insertion type mounting, surface mounting, flexible mounting, tip bonding mounting, controlled collapse bonding mounting and organic multitip mounting.
In recent years, the mounting density of the parts on this kind of printed printed circuit boards has increased year by year, and there is the tendency that the mounting means be automated. In these mounting steps, the automatic soldering is carried out at a high temperature, and therefore the boards are required to be heat-resistant.
Furthermore, in the step of circuit formation, a chemical or thermal change such as etching or washing is given, but it is required in this step that the board maintains an original plane state without curling and without separation between the metallic conductor and film used.
Each of the conventional polyimide flexible metal-clad laminates is mainly formed by causing a polyimide film to adhere to a metallic conductive foil with the interposition of an adhesive. However, since this laminate contains the adhesive, its heat resistance is poor and its electrical properties change with time, and therefore it is difficult to apply conventional boards to a field in which the high-density circuits are automatically mounted on the laminates.
As means for solving these problems, methods for producing a polyimide flexible metal-clad laminate having no adhesive have been disclosed in which there is directly applied, to a metallic conductive foil, a solution of a polyimide precursor having a linear thermal expansion coefficient substantially equal to that of the metallic conductive foil, followed by a thermal cure treatment to form a polyimide film thereon (Japanese Patent Application Laid-open Nos. 61-111359, 63-214185, 58-155790, 63-245988 and 61-111182).
The polyimide flexible metal-clad laminate having no adhesive not only solves the problem of deterioration by the adhesive but also permits the remarkable simplification of a manufacturing process and the decrease of cost. However, in order to prepare this polyimide flexible metal-clad laminate having no adhesive, it is necessary to select the polyimide having a linear thermal expansion coefficient substantially equal to that of the conductor, considering the curl properties of the laminate.
Nevertheless, the thus obtained flexible metal-clad laminate curls immediately after the curing, with the conductor foil facing to the outside. In addition, when the unnecessary portions of the conductor are removed by etching to form a circuit, the polyimide film curls, with the conductor foil facing to the inside, which interferes with the subsequent operation.
As a technique for controlling the curl immediately after the curing, Japanese Patent Application Laid-open Nos. 59-22388 and 59-22389 have disclosed a simple curl prevention method by which a reverse bending plastic deformation is given to the metal. In the case of this method, stress must be forcedly applied to the metal conductor, and thus the metal conductor is fatigued.
In addition, another method for preventing the board from curling has been disclosed in which the laminate is bent in a drying/curing step for the formation of the polyimide, as shown in Japanese Patent Application Laid-open No. 63-74635. In this method, facilities for bending the laminates are necessary in the drying/curing step.
With regard to a curl prevention method after the removal of the unnecessary portions of the conductor by etching, Japanese Patent Application Laid-open No. 63-181395 has disclosed a method in which the surface of polyimide is chemically etched after the removal of the unnecessary portions of the conductor by etching. However, in this method, a processing step is additionally required in addition to a production step. Moreover, Japanese Patent Application Laid-open Nos. 64-82928 and 1-245586 have disclosed an attempt in which a polyimide film is coated with plural polyimide layers having different thermal expansion coefficients to decrease the curl of the polyimide film. However, it is difficult to decrease the curl only by the formation of the plural layers having the different thermal expansion coefficients.
An object of the present invention is to provide a method for producing a flexible polyimide metal-clad laminate having no adhesive by which the above-mentioned problems can be solved and by which the curl of the laminate immediately after curing and the remaining curl of a polyimide film after the laminate etching for removing the unnecessary portions of the conductor can be controlled economically and practically.
Another object of the present invention is to provide a method for producing a printed circuit board from this flexible metal-clad laminate, and an electronic apparatus having the printed circuit board on which electronic parts are mounted.
The present inventors have investigated the curl appearance of a flexible metal-clad laminate which can be obtained in accordance with a method comprising directly applying a polyimide precursor solution onto a metallic conductive foil, and then subjecting the same to a thermal cure treatment to form a polyimide film, and as a result, they have found the following facts.
The first curl takes place at the time of the drying of the applied precursor solution and at the time of the subsequent thermal cure treatment, and the second curl appears at the time of the removal of the unnecessary portions of the metallic conductive foil by etching, and in the latter case, with the metallic conductive foil facing to the inside. The formation of the first curl is due to stress generated by the volume contraction of the polyimide film. However, most of this stress can be relieved by fixing the conductor so that the latter may not be bent at the time of the curing, and by providing a hysteresis or heat history higher than the glass transition point of the polyimide. On the other hand, the formation of the second curl would be due to the following fact: The molecular chains of the polyimide are difficult to move owing to the roughness on the interface between the polyimide film and the metallic conductive foil, so that the stress is scarcely relieved, and in consequence, the remaining stress layer is formed on the interface to form the second curl.
In addition, the degree of internal stress generated in the polyimide film depends upon the roughness on the surface of the metallic conductive foil which comes in contact with the polyimide film. When the roughness is large, the stress increases, and when the thickness of the polyimide film increases, the curl formation is inhibited by stress release after the etching owing to stiffness of the film.
The present invention has been completed on the basis of the above-mentioned knowledge.